The present invention relates to a method and system for communication between a plurality of carriages that travel along rails. More particularly, the present invention pertains to a method and system for performing communication by superimposing signals on electric current that flows through a power cable extended along rails.
In the prior art, a monorail type carriage (movable body) that travels along a rail, which is hung from a ceiling of a factory or a storehouse, to transfer articles between stations is known. Japanese Unexamined Patent Publication No. 6-153305 describes a system for supplying electric power to such a carriage. In the system, a pickup coil provided on the carriage is opposed to a power cable, which extends along a rail, in a non-contacting state. The pickup coil generates induced electromotive force from the alternating current that flows through the cable. The induced electromotive force is supplied to the carriage and used as the electric power that drives the carriage.
More specifically, as shown in FIG. 9, an alternating current of a certain voltage (e.g., 200V) flows through two cables 53 that are supported along a rail 51 by a supporter 52. A carriage obtains electric power through a pickup coil 54 opposed to the two cables 53. The pickup coil 54 includes an E-shaped core 55, a projection 55a projecting from the core 55, and a coil 56 wound about the projection 55a.
The carriage (local station) communicates with a fixed station (key station) located on the ground. The operation of the carriage is controlled by command signals sent from the fixed station. Communication between the fixed station and the carriage may be obstructed by crosstalk when employing a wireless system. Thus, a trolley wire extended along the rails is normally used for communication between the fixed station and the carriage. However, abrasion tends to occur on the trolley wire where it contacts a collecting brush. Furthermore, the wiring of the trolley wire is burdensome.
To solve such problems, the assignee of this application has employed the power cables for communication. As shown in FIG. 10, alternating current supplied by a high frequency power source 62 flows through the cables 53, which are arranged along the rail 51. A carriage 61 travels on the rail 51. Communication signals are superimposed on the alternating current flowing through the cables 53 by transmission antenna coils 64 provided on the fixed station 63. A communication antenna coil 65 (refer to FIG. 11(b)) is wound about a core 66 on each carriage 61. This structure enables communication between the carriage 61 and the fixed station 63.
As shown in FIG. 10, when a plurality of carriages 61 travel along the rail 51, collisions between the carriages 61 must be avoided. Collisions may be avoided by installing a sensor on each carriage 61 to detect objects located in front of the carriage 61 with respect to its moving direction. However, at curved portions of the rail 51, an object in front of the carriage 61 may not be detected until the carriage 61 is in close proximity to the object. In such cases, it may be too late to prevent collision when the object is detected. Therefore, communication is carried out between carriages 61 to confirm the location of other carriages 61 and avoid collisions.
As shown in FIG. 11(a), the antenna coils 64 are directly wound about the cables 53 at the fixed station 63. This decreases leakage flux and minimizes signal attenuation. However, the antenna coil 65 of the carriage 61 cannot be directly wound about the cables 53 since the carriage 61 must travel along the cables 53. Therefore, as shown in FIG. 11(b), the antenna coil 65 is wound about the core 66, which is arranged in the vicinity of the cables 53.
The arrangement of the antenna coil 65 in the vicinity of the cables 53 increases flux leakage and signal attenuation in comparison to directly winding the coil 65 about the cables 53. Hence, in a case such as that shown in FIG. 12, a signal (frequency fO) superimposed on the alternating current, which flows through the cables 53, and transmitted from carriage M to carriage N is weakened by the signal attenuation that takes place when the signal is transferred from the antenna coil 65 of the carriage 61 denoted as M to the cables 53. The signal is further weakened when transferred from the cables 53 to the antenna coil 65 of the carriage 61 denoted as N. This results in poor reliability of communication between carriages 61. Additional attenuation of the signals takes place, especially when using a long rail 51, when the signal transmission distance between carriages 61 is great. This may cause erroneous signals to be received by the recipient carriage 61 and stop the entire system or cause other problems.